The present invention relates generally to systems for providing uninterrupted power in the event utility line power is interrupted and, specifically, to a system for avoiding transients when utility line power is lost or restored.
As the use of electronic equipment such as computers, word processors, printers, etc. becomes more widespread, increasing attention has been focused on the problem of what happens during a utility line power source failure. Such failure can be either a complete loss of power for extended periods of time, or can be a momentary "glitch" which may be a very short duration reduction in the voltage. In many installations, even such a short transient can result in a loss of stored information and/or the introduction of spurious signals into the system logic. Thus, it is desirable to provide an uninterruptable power source to completely preclude even momentary loss of power in the event of a utility line outage.
U.S. Pat. No. 3,050,635, issued to Tanner, on Aug. 21, 1962, describes an Uninterrupted Power Source (UPS) where power is normally supplied to a load by means of an AC motor-generator set. Upon failure of utility power, the AC motor-generator set is driven first by a DC motor powered by a battery and, additionally, by a diesel motor, if needed.
In U.S. Pat. No. 4,203,041, issued to Sachs on May 13, 1980, there is disclosed a battery powered generator set which provides an AC output to the load during the power outage. Sachs utilizes AC and DC machines, each capable of operation as a generator or a motor. During normal utility line operation, the AC machine operating as a motor coupled to the utility line drives the DC machine operating as a generator, which charges a battery power supply. Upon interruption of the utility power, the DC machine operates as a motor powered by the battery power supply, and turns the AC machine operating as an AC generator feeding the load.
The above are examples of motor-generator sets for the purpose of providing an uninterrupted power supply. Additionally, battery powered inverters are known which chop, transform, and filter a battery supply output to obtain an alternating current to provide uninterrupted power in the event of a utility line power outage. However, if the static inverter is operated continuously with the utility only feeding the battery, the inverter reliability itself becomes a problem.
One problem that must be overcome in any uninterrupted power supply system is a transient when the utility outage occurs, or is restored. Differences in phase between the utility line power and the UPS system power result in very high current peaks during reapplication of utility power. In the case of motor generator sets, this high current can result in severe torque disturbances.